It is known in the art to operate hydraulic motors of the type mentioned in the preamle by opening a by-pass conduit between the inlet side and the outlet side of the motor chamber, so that the inlet side of the motor chamber will be pressure unloaded. It is for example known to use this principle in connection with hydraulically driven circular saw discs, cutter discs, and the like. It i s an advantage of this technique that a main valve in the by-pass conduit advantageously can be operated by means of a pilot valve, which needs only a very small actuation power at the same time as the design can be made compact and light, which makes it suitable for use in portable machines. It is, however, a drawback that the operation of the motor by means of a valve in a by-pass conduit does not in fact shut off the conduits to and from the motor chamber but only lead the main flow of hydraulic liquid another way, which means that an idling flow of hydraulic liquid may pass through the motor chamber. This in turn means that it is not ensured that the tool which is powered by the motor, for example a circular sawing disc, a cutter disc, or the like, can be completely stopped by opening the by-pass conduit but continue to rotate. It is true that it will rotate at a smaller force and with a low rate, but however rotate, because of a certain idling flow.